20 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



the first and second polar bodies (Fig. 13 A-D). The second maturation 

 spindle and second polar cell are smaller than the first. Immediately 

 after the formation of the second polar cell, the chromosomes resolve them- 

 selves into a reticulum and the female pronucleus is formed (Fig. 13 D). 



Maturation of the Human Ovum. The only observations are those 

 of Thompson (1919), who believes to have identified stages in the forma- 

 tion of all three polar cells prior to ovulation or fertilization. The evi- 

 dence presented, however, can hardly be accepted as conclusive. 



FERTILIZATION 



The stimulus initiating development in most multicellular animals 

 is furnished by a spermatozoon which penetrates the ovum and fuses with 

 its nucleus. These events constitute fertilization. 



Only motile spermatozoa are able to attach to the surface of an egg; 

 it is probable that forces allied to phagocytosis, rather than vibrational 

 energy, accomplish the actual 'penetration.' Spermatozoa may enter 

 the mammalian ovum at any point. If fertilization is delayed too long 

 after ovulation, the ovum may be weakened and allow the entrance of 

 several spermatozoa. This is known as polyspermy. In such cases, how- 

 ever, only one spermatozoon unites with the female pronucleus. 



The fundamental results of the process of fertilization are: (i) the 

 union of the male and female chromosomes to form the cleavage nucleus of the 

 fertilized ovum; (2) the initiation of cell division, or cleavage of the ovum. 



These two factors are separate and independent phenomena. It has been shown by 

 Boveri and others that fragments of sea urchin's ova containing no part of the nucleus may 

 be fertilized by spermatozoa, segment, and develop into larvas. The female chromosomes 

 are thus not essential to the process of segmentation. Loeb, on the other hand, has shown 

 that the ova of invertebrates may be made to develop by chemical and mechanical means 

 without the cooperation of the spermatozoon (artificial parthenogenesis) . Even adult frogs 

 have been reared from mechanically stimulated eggs. It is well known that the ova of 

 certain invertebrates develop normally without fertilization, that is, parthenogenetically. 

 These facts show that the union of the male and female pronuclei is not the means of initiat- 

 ing the development of the ova. In all vertebrates it is, nevertheless, the end and aim of 

 fertilization. 



Lillie (1912; 1913) has recently shown that the cortex of sea urchin's ova produces a 

 substance which he terms fertilizin. This substance he regards as an amboceptor essential 

 to fertilization, with one side chain which agglutinates and attracts the spermatozoa, and 

 another side chain which activates the cytoplasm and initiates the cleavage of the ovum. 

 According to Loeb (1916), agglutination is proved in but few forms and Lillie's interpreta- 

 tion fails to meet all the facts. Loeb (1913) holds that the spermatozoon actually acti- 

 vates the ovum to develop by increasing its oxidations and by rendering it immune to the 

 toxic effects of oxidation. 



Fertilization of the Mouse Ovum. -Normally, a single spermatozoon 

 enters the ovum six to ten hours after coitus. While the second polar cell 

 is forming, the spermatozoon penetrates the ovum and loses its tail. Its 



